During flight preparation or during a rerouting, the crew stores their flight plan on a dedicated computer, known by the name of Flight Management System or FMS.
The flight plan is defined by the pilot as being a set of pairs made up of a segment and of its final point; each pair is also called a Leg. The trajectory is computed as a function of the segments and of their final points as well as of the altitude and speed conditions (which are used in particular for the computation of the radius of the circular segments).
For various reasons, the pilot may choose to shift the trajectory laterally by a distance and by a direction of shift which is determined, the direction being defined as being a shift to the right or to the left with respect to the direction of the aircraft. These operational reasons are:                Lateral avoidance of a dangerous zone (cumulo nimbus, mountains);        Procedure making it possible, in a zone where the air traffic control service is cut off, to laterally separate aircraft that are following one another or crossing one another;        Lengthening of the flight plan so as to perform a synchronization with other aircraft, or to ensure the achieving of a time constraint applied to on a point of the flight plan;        Management of an onboard communication fault (faulty radios); in this case, by procedure, the aircraft must be shifted laterally onto an unoccupied corridor.        
A method commonly called lateral shifting or lateral offset is known in the prior art, making it possible to cover part of this need. However, this procedure is not suited to the whole set of segments defined in the Arinc 424 standard. It applies only for the segments of type TF, CF, FM or DF.
SegmentNameMeaningIFInitial FixFixed initial point on the groundCFCourse To a FixProceed/Follow a ground track to a fixed pointDFDirect to a FixProceed direct (straight) to a fixed pointTFTrack between two FixesGreat circle between 2 fixed pointsAFArc DME to a FixDefines a circular arc around a specified remoteDME beacon, with an aperture limit.RFRadius to a FixDefines a circular arc between 2 fixed points (the 1stpoint being the fix of the preceding segment), on acentre of the fixed circle.VIHeading to InterceptDefines a heading to be followed until interceptionof the following segmentCICourse to InterceptDefines a course to be followed until interception ofthe following segmentVAHeading to AltitudeDefines a heading to be followed until a givenaltitudeCACourse to AltitudeDefines a course to be followed until a givenaltitudeFAFix to AltitudeDefines a course to be followed, starting from afixed point, until a given altitudeVDHeading to DME DistanceDefines a heading to be followed until interceptionof a specified DME arcCDCourse to DME DistanceDefines a course to be followed until interception ofa specified DME arcVRHeading to RadialDefines a heading to be followed until interceptionof a specified radialCRCourse to RadialDefines a course to be followed until interception ofa specified radialFCTrack from Fix to DistanceDefines a course to be followed starting from a fix,over a specified distanceFDTrack from Fix to DMEDefines a course to be followed starting from a fix,Distanceuntil intercepting a DME arc (specified DMEdistance)VMHeading to ManualDefines a heading without termination (infinite halfline)FMFix to ManualDefines a course, starting from a fix, withouttermination (infinite half line)HARacetrack pattern, with Altitude exit conditionHFRacetrack pattern, with a single turnHMManual racetrack pattern, without exit conditionPIFix to ManualOutbound procedure defined by an outboundcourse starting from a fix, followed by a half turn,and interception of the initial outbound course forthe return.
Indeed, the sequences of segments of this type are deterministic, and the lateral shift is simple to compute.
FIG. 1 presents the method of shifting a segment 101 of type TF, CF, FM or DF, in accordance with the prior art and by a shift distance d. In this case the shifted segment 102 is determined by a first step during which the final point 103 is shifted by the shift distance along the bisector between the segment 101 and the following segment 104, so as to create the shifted termination point 105. Finally, the shifted segment 102 is determined so as to be of the same type as the initial segment 101 and to finish at the shifted final point 105.
FIG. 2 presents the method of shifting an initial segment 101 of type IF. In this case the shifted segment 102 is determined by a first step during which the initial termination point 103 is shifted along the perpendicular to the successor segment 104 of the said initial segment, so as to create the shifted final point 105. The shift is performed by the shift distance and along the direction of shift. Lastly the shifted segment 102 is determined so as to be of the same type as the initial segment 101 and to finish at the shifted final point.
FIG. 3 presents the method of shifting a segment 101 of racetrack type (HA, HF, HM). This special segment has the particular feature that its final point is the same as the final point of the predecessor segment. It is therefore possible to use the shifted final point of the predecessor segment (entry point) and to thereafter construct the shifted segment (the racetrack) with the same geometric characteristics (track, length, Right/Left side) as the initial segment. Moreover, during the computation of the position of the segment, if the successor (respectively preceding) segment is of type HA, HF or HM then the segment which succeeds (respectively: which precedes), the successor segment must be considered in its place. During the construction of the shifted trajectory, when the preceding (respectively following) segment is a segment of type HA, HF or HM, then the segment preceding (respectively following) the segment of type HA, HF, HM is considered for the computation of the bisector or of the perpendicular, the segment of type HA, HF, HM is however ignored by the computation of the shifted final point associated with the segment.
FIG. 4 presents the method of shifting a segment 101 of type CI, VI. The shifted final point 105 associated with the shifted segment 104 is computed by the customary methods, but starting from the shifted position of the preceding segment and considering that the segment 104 immediately succeeding the initial segment has been shifted laterally to give a new segment 401 immediately succeeding the shifted segment.
FIG. 5 presents the method of shifting a segment 101 of type CR or VR. In this case the shifted final point 105 associated with the segment CR or VR is computed by the customary procedures of the prior art, but laterally shifting the reference radial 501 by the shift distance and along the direction of shift so as to create a shifted reference radial 502.
FIG. 6 presents the method of shifting a segment 101 of type CD or VD. The shifted final point 105 associated with the shifted segment 102 of type CD or VD is computed by the customary procedures of the art, but shifting the reference beacon 601 by the shift distance perpendicularly with respect to the direction of the initial segment 101 (the reference beacon represents the centre of the circle) of the segment CD or VD in the sense of the shift so as to obtain a shifted reference beacon 602.
FIG. 7 presents the method of shifting a segment 101 of type FA. The shifted segment 102 is computed by laterally displacing the initial termination point 103 associated with the initial FA segment on the perpendicular to the direction of the said initial segment. The shift is performed on the right part with respect to the aircraft if the direction of shift is to the right and on the left part if the direction of shift is to the left. If the reference point of the segment of type FA is common with the preceding point, then the shift logic for the preceding point applies. Indeed, in the case for example of a sequence made up of a segment of type CF followed by a segment of type FA where the termination of the segment of type CF is the same as the initial point of the segment of type FA. It is therefore possible to use the shifted final point of the segment of type CF to construct the shifted type FA segment.
FIG. 8 presents the method of shifting a segment 101 of type PI. This shifted segment 102 is computed on the basis of the shifted position of the final point, since the start of the segment of type PI is always common with the final point associated with the preceding segment. The computation of its termination being done with the commonly used logic.
In the case of the first segment of a flight plan, the determination of the first shifted segment begins with the computation of the shifted position of the first final point of the said segment. In the prior art, this position is computed in the following manner:                If the second segment of the flight plan is a segment of type TF, then the shifted final point is defined as being on the perpendicular of the departure track of the TF segment from the initial termination point and at a distance corresponding to the shift distance from the original final point.        If the second segment of the flight plan is a segment of FM type then the shifted final point is defined as being on the perpendicular of the departure track of the segment of FM type from the initial termination point and at a distance corresponding to the shift distance from the original final point.        If the second segment of the flight plan is a CF segment then this position is not necessary.        If the second segment of the flight plan is a DF segment then the DF segment is constructed as a CF using the track of the previously computed DF and this position is not necessary.        
However, in the operational cases explained hereinbelow, the current method does not make it possible to perform the lateral shift (since the current state of the art does not make it possible to perform a shift for a flight plan exhibiting certain types of segments):                In lateral flight plans with performance constraints, known by the name of Required Navigation Performance or RNP, the RF and AF segments are designed to manage the turns in a deterministic manner. Now, the current function does not make it possible to solve these cases.        In the case of circular segment of RF or AF type.        Lastly, future functionalities such as the relative positioning between aircraft, known by the term ASAS, are not compatible with a lateral shift with the current function.        